1. Field of the Invention
The present invention relates to a birefringence measurement optical system for measuring the birefringence of a sample such as a DVD (Digital Video Disc), a liquid crystal display and the like and relates to a high spatial resolution polarimetric apparatus. The present invention particularly relates to a device of an optical constitution which can be polarimetrically analyzed with high spatial resolution.
2. Description of the Related Art
In recent years, there is strong demand for a high-density recording medium and a high precision display device as a constituent technique in the field of rapidly developing multimedia. A DVD, for example, is intended to realize a high-density recorded signal by providing material with low birefringence and high qualilty. A liquid crystal display is intended to realize high precision by transferring SVGA (Super Video Gate Array) to XGA (Extended Graphics Array).
Among material evaluation techniques to obtain these objects, attention is attracted by, in particular, a quantitative birefringence measurement technique for measuring the state of birefringence in the vicinity of a signal pit for, for example, a DVD, birefringence information in pixel units for a liquid crystal display or the like on a microscopic scale.
As such birefringence measurement apparatuses, there have been conventionally known a polarizing microscope and a polarimetric apparatus. The polarizing microscope, however, employs an optical microscope incorporating a polarizer and a phase plate, with which microscope, the state of a birefringence distribution is observed qualitatively and intuitively with a magnification according to a combination of an objective lens and an eyepiece. The polarizing microscope is not, therefore, available for the quantitative measurement stated above.
The polarimetric apparatus adopts, for example, a method using a rotating analyzer, a phase compensation method, a Senarmont method, a phase modulation method, an optical heterodyne method or the like. Since this apparatus is designed to receive the overall light fluxes passing through the sample at the photo detector, the spatial resolution is determined by the size of diameter of the light fluxes and the positional resolution for measurement is limited to about 0.5 mm. For these reasons, it is difficult to use the apparatus for microscopic measurement as it is.
Considering the above disadvantages, as one of the approaches for quantitatively, microscopically measuring birefringence, a method of converging a light signal on a sample using a convergent lens which is added to the optical system of the above-stated polarimetric apparatus or the like is desired. In this method, a beam can be passed though an extremely limited region of the sample and it is, therefore, possible to further enhance spatial resolution.
In the polarimetric apparatus using the convergent lens as stated above, although spatial resolution on the sample enhances, light fluxes within a light beam have different incidence angles with respect to the sample. Due to this, the following disadvantages occur.
FIGS. 32A to 32C are views for describing the variation state of birefringence depending the difference in incidence angle. The amount of birefringence of the sample such as an optically anisotropy crystal or a polymer material is considered while using a three-dimensional refractive index ellipsoid determined by the refraction indices nx, ny and nz on the X, Y and Z axes, respectively. If so, the amount of birefringence varies according to observation direction, that is, the difference of refraction index between the major axis and minor axis of the ellipsoid in a plane passing the origin of the refractive index ellipsoid and perpendicular to the observation direction occurs.
In other words, the direction of the refractive index ellipsoid differ, between a case where parallel light is vertically incident on the sample as shown in FIG. 32A and a case where parallel light is obliquely incident on the sample as shown in FIGS. 32B and 32C. As a result, there occur different phase differences of birefringence received by light differ as in the case of the different observation directions as stated above.
Then, consideration is given to a case where convergent light is incident on a sample using a lens system as shown in FIG. 33. In this case, light fluxes A and B having different incidence angles and positioned in the convergent light, for example, receive different retardations from the sample. Due to this, the amount of retardation received by the overall convergent light is the product of the phase differences received by respective very small light fluxes divided from the light flux. Thus, if convergent light is incident on a sample, the following disadvantages occur, i.e., a light incidence angle cannot be specified in a strict sense and the amount of birefringence of the sample cannot be precisely measured.